As used herein, “retinal retroflection” or “RR” means reflection in which (i) a camera and a light source subtend an angle, from the vantage point of an eye, that is less than eight degrees; and (b) light from the light source travels to the retina of the eye, and then scatters from the retina, such that a portion of the scattered light travels to the light source and a portion of the scattered light travels to the camera.
Retinal retroflection occurs when the light source is sufficiently near the camera. This is sometimes loosely described as the camera being coaxial with the illumination. If the light source and camera are too far apart (i.e., subtend an angle, from the vantage point of the eye, that is greater than eight degrees), then the camera typically does not capture an image of retroflection from the retina.
An example of retinal retroflection is a red reflex reflection that occurs when the light source emits visible light. If the visible light includes reddish-orange light, then the red reflex reflection appears, from the vantage point of the camera, to be a reddish-orange region in the pupil of the eye. (This is a familiar phenomenon. Many camera owners have seen a red reflex effect, in which a person's pupils appear red or orange in a photograph.)
Another example of retinal retroflection is a bright pupil reflection that occurs when the light source emits infrared (IR) light or near-infrared (NIR) light. A bright pupil reflection is sometimes called a white pupil reflection.
The retinal retroflection (RR) normally appears approximately circular in shape. However, the shape of the RR may vary depending on eye conditions such as refractive aberrations or pathologies that occlude or diffuse light or that prevent reflection from the retina.
In retinal retroflection: (a) the angular distance between the camera and light source—as seen from the vantage point of the eye—is very small; (b) the eye converges light from the light source to form a blur spot on the retina; and (c) light scatters from the blur spot and travels to the camera.